Red tide is a world-wide phenomenon and is the sudden appearance of marine organisms that produce discolored water and various problems. In Florida the red tide organism (P. brevis) Gymnodinium breve is associated with mass marine fatalities, with economic costs estimated at twenty million dollars (for a three-month outbreak) and with environmental health hazards. These include respiratory problems that may affect thousnads who live, shop, or work near beach communities. In addition, skin irritation may affect others, and some persons face various degrees of personal discomfort if they consume oysters or clams that mechanically entrapped P. brevis cells. A cynaobacterium--Gomphosphaeria aponina-- isolated from a red tide outbreak produces a material ("aponin" that is cytolytic toward P. brevis). Aponin or G. aponina could be used to develop preventative medical strategies to aleviate the environmental health hazards associated with red tide. Specifically, patches of red tide could be seeded with G. aponina or treated with aponin and an outbreak could be controlled at early stages befor massive spreading. To this end, four major goals are recognized. (1) Though purification of the cytolytic material has been achieved and though much useful information has been obtained, a suitably crystalline sample of the purified substance is a high priority. (2) The achievement of long-term preservation of G. aponina so that concentrated samples would be available for on-site application during a red tide. (3) Evaluation of the correct concentration and correct protocol for causing P. brevis to go into a resting stage rather than lyse. (Airborne particles from lysed G. breve cells seem to be responsible for respiratory problems, and aponin can induce the resting stage of P. brevis in laboratory cultures.) (4) To evaluate the mode of action of purified aponin. The activity of aponin and G. aponina toward P. brevis is a continuation of an earlier study aimed at reducing the health hazards of red tide outbreaks.